This invention relates to a method for generating localized heat within materials and biological tissue by the means of intersecting beams of ultrasound.
High frequency acoustic waves, or ultrasound, may be used to remotely heat industrial or biological materials. There has been strong evidence in research and clinical laboratories that focused ultrasound for cancer hyperthermia will become a useful mode of treating cancer patients, in addition to the surgical, radiological and chemotherapeutic methods that are available now. In the treatment of tumors in cancer hyperthermia, focused ultrasound heats the tumor to a temperature of approximately 43.degree. C. while the adjacent healthy tissue is kept at a lower temperature closer to normal body temperature (37.degree. C.). The elevated temperature in the tumor disrupts the tumor growth and eventually kills it. This allows the cancer to potentially be treated without surgery, without ionizing radiation, or without chemotherapy.
Conventional focused ultrasound for heating is employed by using either a scanned ultrasound transducer or with a phased array. The scanned transducer uses a lens, much like an optical magnifying glass focuses sunlight, while the phased array uses electronic delays among the array elements to achieve focusing. A burst of sound is then emitted which converges at the focus to provide localized high intensity acoustic energy. Some of the high intensity acoustic energy is absorbed by the tissue at the focus and is dissipated as concentrated focal heat. The rest of the energy travels through the focus and is slowly dissipated into the surrounding tissues as distributed heat.
Biomedical hyperthermia applicators using a plurality of sound sources to heat larger, distributed volumes, have also been investigated. These investigations have relied upon linear thermal superposition of the plurality of sound sources to heat the target tissues. Nonlinear effects of sound propagation through animal tissue and materials have also been studied for a single sound source.
The nonlinear mixing, or intermodulation, of sound waves has been known in oceanographic acoustics. Oceanographic acoustic applications have used both the linear (superposition) and nonlinear (intermodulation) effects of intersecting sound beams. Nonlinear acoustic sonars, known as oceanographic parametric sonars, deliberately promote the generation of a difference frequency to enhance sonar beamforming and long range sound propagation. The generated difference frequency is usually 30 to 60 dB below the level of the primary frequencies. A second product of nonlinear mixing is the sum frequency, which is generated by the intermodulation process at 10 to 40 dB below the level of the primary frequencies, indicating that the conversion from primary to sum frequency is a significantly more efficient process than the conversion of a primary to a difference frequency. Since higher frequencies are subject to higher absorption coefficients in water they generate more heat than the primary frequencies as they propagate, but propagate shorter distances than the primary frequencies. In oceanographic sonar applications, heat generation via sound absorption is generally an undesirable result of nonlinear intermodulation.